An evaporator that carries out heat exchange between exhaust gas of an internal combustion engine and water so as to heat the water by the heat of the exhaust gas and generate high temperature, high pressure steam is known from Japanese Patent Application Laid-open Nos. 2001-207910 and 2001-207839.
Japanese Patent Application Laid-open No. 2001-207910 discloses an arrangement in which an evaporator is disposed in each of a plurality of exhaust ports of a multicylinder internal combustion engine, thus giving a high efficiency of heat exchange with a high temperature exhaust gas while avoiding the occurrence of exhaust interference and thereby ensuring the output of the internal combustion engine, and a single evaporator is disposed in a section where a plurality of exhaust passages are combined, thereby improving the efficiency of heat exchange by using exhaust gas that has decreased pulsations after being merged and thus has a uniform temperature. Japanese Patent Application Laid-open No. 2001-207839 discloses an arrangement in which a plurality of heat exchangers are disposed in a layered state in an exhaust passage of an internal combustion engine, thus lowering the heat transfer density of a heat exchanger on the upstream side where the flow rate of exhaust gas is high and increasing the heat transfer density of a heat exchanger on the downstream side where the flow rate of exhaust gas is low, and thereby ensuring a uniform heat transfer performance across all of the heat exchangers.
However, since the above-mentioned conventional heat exchanger has a structure in which heat exchange is carried out by contacting the exhaust gas with the external surface of a spiral- or zigzag-shaped pipe member within which water flows, the heat transfer area is limited to the surface area of the pipe member, and there is a limit to the improvement of the heat exchange efficiency.